If you want to test a new vaccine, heart valve or intravenous drug, you need a horseshoe crab, or, specifically, you need horseshoe crab blood. The bizarrely light blue vascular fluids of these curious marine creatures are packed with amebocytes, cells which produce an almost instantaneous defensive reaction to toxins, making them perfect for trialling new medical developments under the so-called LAL (Limulus amebocyte lysate) test. Consequentially, over half a million horseshoe crabs are plucked from their native habitats around the North American Atlantic coast every year (559,903 in 2015), each one ‘bled’ of up to 400ml of blood before being returned to the ocean (significantly far enough away from where they are initially harvested to limit the chances of animals being bled repeatedly).

The bleeding of horseshoe crabs is seen as vital for medicinal research (Image: Alamy)

The survival of the horseshoe crab is now threatened by this harvesting process. Somewhere between ten and 30 per cent of the animals die as part of the process, partially through blood loss, but primarily as a result of stress, high temperatures, accidental damage caused by being handled, or by developing hypoxia from spending significant periods of time out of the water. The Atlantic States Marine Fisheries Commission sets a maximum harvest mortality limit of 57,500 crabs per annum, but this figure is regularly exceeded. Furthermore, harvesting during spawning season makes it especially hard for the species to repopulate. The overall impact has been dramatic, with some coastal areas seeing horseshoe crab populations crashing, in the most extreme cases, by as much as 95 per cent over 15 years.

‘The gradual extinction of this once-abundant important species is alarming, and evidence shows many migrating bird populations have been declining in tandem with diminishing horseshoe crab populations,’ says Dr Anthony Dellinger from the University of North Carolina at Greensboro. ‘We wanted to find out the scope of the problem and begin to assess alternatives to current harvesting procedures.’ Farming captive horseshoe crabs could reduce the need for wild harvesting of the species, and make the whole LAL testing process more sustainable, explains Dellinger, while some researchers maintain hopes of even substituting a synthetic solution that requires no harvesting of the crabs at all. Nevertheless, separate threats, including climate change, habitat destruction, and the use of horseshoe crabs as eel and whelk fishing bait, look set to heap further pressure on this strange but vital species.

This was published in the September 2018 edition of Geographical magazine

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